Equipment for rock drilling emits noise of high level. Specific for such types of equipment, the noise emanates on the one hand from the hammer drilling machine, in top hammer drilling as well as in down-the-hole drilling, on the other hand from the components of the drill string. During, percussion drilling, the most important source of noise generation is the bending vibrations in the rod elements, the impact rods, caused by their instantaneous bending which is induced in connection with the impacts. Hereby the bending is in general caused by the drill bit at impact meeting an uneven opposing substrate.
When the impact wave or the shock wave from the hammer reaches the drill bit, a bending moment thereby occurs because the drill bit meets different resistance at its different front portions.
In common for equipment of these kinds is that they are subject to strict restrictions as concerns the level of emitted noise to be used in connection with other activities.
For the purposes of limiting noise emissions from drill rigs and associated components i.a. has been suggested to isolate the percussive drilling machine and the drill string from the surroundings through noise damping housings. This solution, however, brings along a number of disadvantages such as extra costs and reduced flexibility during drilling. Further, the operator will be shielded from the components, giving inferior possibilities of operation control.
Another solution that has been tested is to provide the elements of the drill string with friction rings, which, however, have, had limited effect and which have been difficult to keep in position.
Considerations concerning reduction of noise from drill strings have resulted in suggestions to redesign involved elements, for example through reduction of tolerances such that the bending vibrations would be eliminated or at least reduced.
As examples of background art can also be mentioned: “Int. J. Rock Mech. Sci. & Geomech. Abstr. vol. 16, sid. 363-376; Pergamon Press Ltd 1979, GB; Investigation of Noise and Vibration in Percussive Drill Rods”, “Inter-noise 1986 July, 21-23 Cambridge US; Concentric drill steels for noise reduction of percussion drilling, (Stein, Aljoe)”, U.S. Pat. No. 3,926,265, U.S. Pat. No. 4,168,754, U.S. Pat. No. 4,591,009, U.S. Pat. No. 6,427,782B2, U.S. Pat. No. 7,210,555B2, US2005/0279565A1.